Question: In chemistry class, the girl to boy ratio is $9$ to $6$. If there are $30$ boys, how many girls are there?
A ratio of $9$ girls to $6$ boys means that there is a group of $9$ girls for every group of $6$ boys. If there are $30$ boys, then there are $5$ groups of $6$ boys each. Because we know the ratio is $9:6$ , there must be $5$ groups of $9$ girls each. There is a total of $45$ girls in chemistry class.